Mechanical seals are often used in the shaft sealing apparatus of rotary machines to convey fluids. However, despite the fact that mechanical seals have an extremely high probability of failure compared with other mechanical elements, the occurrence of the abnormal condition cannot be detected until leakage actually occurs. Failure is difficult to predict.
Hitherto, there has been proposed a method whereby a high frequency vibration sensor is attached to the member subject to slippage (on the fixed side) of the rotaty machine and the high frequency vibration which is generated by the slippage is detected, thereby detecting the abnormal condition of the member subject to slippage (Japanese Patent Disclosure No. 55-98353). However, according to this method, the occurrence of an abnormal condition is simply detected by comparing the magnitude of the amplitude of the detected waveform. Therefore, for example, this method is effective to distinguish between the state of contact or noncontact between the balance disk and the balance sheet, between the labyrinth seal and the shaft, and the like. However, it is impossible to check and evaluate the state of constant contact and slippage such as in a mechanical seal.
There has also been proposed a method whereby the vibration of the mechanical seal is measured by a vibration sensor having sensitivity in the relatively low frequency band, and thereby evaluating the slippage of the mechanical seal. However, according to this method, the vibration sensor is easily influenced by the noise of the bearings, fluid sounds, and other sources, other than the mechanical seal, so that the slippage of the mechanical seal cannot be sufficiently evaluated.